Conventional p-n junction diodes and Schottky barrier diodes are associated with a forward voltage threshold. More particularly, an ideal diode would conduct a current in one direction without power loss and would block a current in the opposite direction. However, a practical (real) diode conducts a current in the forward direction, but only after a certain threshold voltage is reached. Furthermore, due to the internal resistance of an actual diode, an additional voltage drop occurs depending on the values of the internal resistance and the current. The sum of the threshold voltage and the voltage drop caused by the internal resistance is referred to as a forward voltage drop (Vf) of a diode.
In circuits having high forward currents, like power rectifiers and OR-ing diodes in redundant power supplies, the power loss can be very high and has a significant impact on the efficiency of the circuit. Therefore, there is a need for a circuit which simulates the function of a diode in that it conducts current in one direction and blocks current in the opposite direction, but has a low forward voltage drop Vf in order to reduce power loss in circuits.
Edlund, in U.S. Pat. No. 4,417,164 issued Nov. 22, 1983 discloses an electrical model for a unidirectional mechanical valve. Edlund notes that the properties of an actual diode for use in such a model has major drawbacks which differ from an ideal diode such as the presence of a voltage drop across the diode when it is conducting current. Further, an actual diode will not begin to conduct current until the voltage difference between the anode and the cathode reaches a turn-on voltage of about 0.5 volts. Additionally, a diode does not change from a non-conducting to a conducting state immediately, but rather has a finite switching speed which depends in part on the rate of the change of the voltage across the diode. A diode is adversely effected by the junction capacitance between the anode and the cathode. Accordingly, Edlund discloses a circuit as shown in FIG. 1A incorporating a field effect transistor (n-channel enhancement MOSFET) 1 with drain and source terminals connected to input and output terminals A and K, respectively, as shown. A voltage comparator 2 has positive input 3 and negative input 4 connected to the drain and source terminals of the MOSFET 1, respectively, as shown. The comparator output 5 is coupled to the gate of the MOSFET 1. The power supply for the voltage comparator is floating so that the device is unaffected by the remainder of the electrical system.
It is known that in an n-channel device, the conventional flow of drain current is in the positive direction--that is, current flows from the drain to the source with a positive gate-to-source voltage. Typically the drain is connected to a higher voltage than the source. Further, it is known that an n-channel device has an integral reverse rectifier associated therewith. This intrinsic diode is an integral part of the device and is not a separate electrical component. In an n-channel device, the intrinsic diode effectively has an anode connected to the source and a cathode connected to the drain.
FIG. 1B is provided by the Applicant for a patent for the present invention for analysis of the actual operation of the prior art circuit of FIG. 1A and shows n-channel enhancement MOSFET 1 having an intrinsic diode 1a with its anode connected to the source and its cathode connected to the drain. The intrinsic diode 1a is enclosed within a circle with the symbol for the MOSFET to indicate that the intrinsic diode 1a is part of the MOSFET 1 and is not a separate electrical component.
The theoretical operation of Edlund's circuit in FIG. 1A is a follows: when a higher voltage is placed on the drain then on the source, the output 5 of voltage comparator 2 goes high and is input to the gate of the MOSFET 1. Accordingly, the MOSFET 1 is controlled to conduct current from the terminal A at a higher voltage to the terminal K at a lower voltage. On the other hand, theoretically, during operation, when the terminal K is placed at a higher voltage than the terminal A, the output 5 of voltage comparator 2 goes low and is input to the gate of the MOSFET 1 in order to control the MOSFET to stop conducting current. Theoretically, the MOSFET 1 will conduct current from terminal A to terminal K when terminal A is at a higher voltage than terminal K. Further, theoretically, in operation, the MOSFET 1 will not conduct current from terminal K to terminal A when terminal K is at a higher voltage than terminal A. Accordingly, theoretically, Edlund's circuit simulates the action of a diode.
In actuality, however, the Applicant for a patent for the present invention has identified a problem in the prior art: when Edlund's circuit is actually constructed employing an actual MOSFET 1, it fails. FIG. 1B is provided to analyze the actual operation of the prior art circuit of FIG. 1A. More particularly, when the voltage at terminal A is higher than the voltage at terminal K, the output 5 of the voltage comparator 2 goes high and is input to the gate of the MOSFET 1. The gate controls the MOSFET to conduct current from terminal A to terminal K. On the other hand, when the voltage at terminal K is higher than the voltage at terminal A, the voltage comparator 2 delivers a low output voltage 5 which is input to the gate of the MOSFET 1 in order to control the MOSFET 1 to stop conducting current and block current from terminal K to terminal A. Due to the integral reverse intrinsic diode 1a of the MOSFET 1, the MOSFET 1, in practice, actually conducts current from the high voltage at terminal K to the low voltage at terminal A. Accordingly, Edlund's circuit shown in FIG. 1A does not actually operate to simulate the function of a diode because it does not take into consideration the effect of the intrinsic diode 1a of the MOSFET 1 as identified by the Applicant for patent for the present invention.
Accordingly, there is still a need for a circuit that functions like a diode in that it conducts current in one direction and blocks current in the opposite direction, however, has a low forward voltage drop. There is also a need for a circuit that functions like a diode in that is conducts current in one direction and blocks current in the opposite direction that reduces power losses.